In the case of a motor vehicle fitted with an automatic multi-step shift transmission such as an automated change-speed transmission, an automated dual-clutch transmission or a converter automatic transmission, after driving onto a downward-sloping stretch a lower gear is engaged, in order to increase the speed of the drive motor which, after the transition to thrust operation, is operating in a switched-off thrust mode, i.e. without fuel injection. Owing to the higher speed of the drive motor, the drag torque and hence the braking action of the drive motor increase. Furthermore, the braking force produced on the driven wheels by the drag torque of the drive motor is additionally increased due to the higher transmission ratio.
If the motor vehicle has a permanent brake such as an electromagnetic or hydrodynamic retarder, as is usual in heavy utility vehicles, this too is additionally activated when driving on steep and/or longer downhill stretches, this being done manually by the driver or automatically by an electronic control unit. In the case of a permanent brake on the primary side, i.e. one arranged on the input shaft of the multi-step shift transmission, at the higher speed of the drive motor both its braking action and its braking capacity, which is limited by the cooling, increase. In contrast, with a permanent brake arranged on the secondary side, i.e. on the output shaft of the multi-step shift transmission, at the higher speed of the drive motor only its braking capacity increases provided that the cooling of the permanent brake is coupled with the cooling of the drive motor, as is the case for example with a secondary retarder of the type known as an ‘intarder’, where the oil circuit is coupled with that of the multi-step shift transmission and the cooling water circuit connected, via a heat exchanger, with the oil circuit is coupled with that of the drive motor. A corresponding automatic downshift takes place as a function of appropriate operating parameters such as the road inclination, the mass of the vehicle and the driver's braking intention.
A corresponding transmission control system is known from DE 33 34 718 A1. This transmission control system comprises devices for determining the road inclination (uphill or downhill inclination of the road), the mass of the vehicle, the engine torque, and a constant torque required for driving the motor vehicle without any acceleration. By means of a downhill gear assignment system an optimally adapted gear is assigned to each downhill road gradient, in which the vehicle can drive down the slope at constant travel speed without additional actuation of the service brakes, and in this the use of a permanent brake, if present, is taken into account. A shift to the optimum gear is either recommended to the driver, for example, by means of a corresponding best-gear indication, or initiated automatically. A shift to the optimum gear is prevented if, while driving downhill, the driver actuates the accelerator pedal or if the gradient has just become clearly smaller.
Furthermore, in U.S. Pat. No. 5,231,897 a corresponding control unit for an automatic transmission is known, in which the current driving resistance is calculated from the vehicle's acceleration and the torque of the drive motor. If the current driving resistance exceeds an upper, first limit value, uphill driving is recognized and an upshift is prevented. If the current driving resistance falls below a lower, second limit value, downhill driving is recognized and a downshift is carried out in order to produce a braking action by the drive motor.
In DE 44 40 706 C2 a method for controlling an automatic transmission is proposed, in accordance with which, from the road inclination detected and the road-stretch profile determined, an effective hill factor is determined which is used as an interpolation factor. When uphill driving is recognized the shift characteristics currently used are determined in each case by an interpolation between a shift characteristic for driving on level ground and a shift characteristic for driving on a steep uphill road. If downhill driving is recognized, the shift characteristics currently used are determined by interpolation between a shift characteristic for driving on level ground and a shift characteristic for driving on a steep downhill road.
According to the prior art an upshift only takes place after driving onto level ground or after the transition to traction operation, i.e. when the driver actuates the accelerator pedal or when an active speed regulating unit has automatically switched on the fuel injection of the drive motor. As an exception, an upshift can be carried out automatically when driving on a downhill stretch as a protective shift, if the speed of the drive motor has reached or exceeded a critical upper speed limit, for example if the vehicle has moved onto a very steep downhill stretch or because of a high vehicle mass. The disadvantage of an upshift that takes place only after moving onto level ground or after the transition to traction operation, is that the momentum of the motor vehicle, i.e. the driving force due to the slope, is used only insufficiently when moving off the slope to propel the motor vehicle while it is moving off the slope and at the beginning of the subsequent driving on level ground or on a new uphill stretch. Due to this the thrust operation ends relatively early and fuel injection is switched on again, which results in unnecessarily high fuel consumption. Furthermore owing to the high engine speed caused by the lower gear, the noise level produced by the motor when moving off the slope is disadvantageously high.